Poly(N-isopropylacrylamide ) ( NIPAAm) gels were formed by photopolymerization of NIPAAm in the absence of a crosslinker using a water solvent at 25ะC. Factors affecting formation were the wavelength region of irradiated light, the type of photoinitiators, and the concentrations of the photoinitiator and monomer. A highpressure mercury lamp (400 W) was used as a light source. An NIPAAm concentration of 10 wt % and irradiation time of 15 h was used for the photopolymerization. The gel ( 68% yield ) was formed when the quartz glass system was used, but no gelation was observed for the Pyrex glass system that transmits light with l รบ 290 nm. The gel ( 100% yield) was easily formed, even in the latter system, when 30 mmol /L of hydrogen peroxide and potassium persulfate were used as the photoinitiator. Water soluble photoinitiators such as ferric chloride and sodium anthraquinone-2,7-disulfonate were not effective for the gel formation. Yield of the gel increased with increasing the potassium persulfate concentration (1 -30 mmol /L), but it decreased when a high concentration of hydrogen peroxide (60 mmol /L) was used. The gel yield increased with the NIPAAm concentration (5 -20 wt % ). The degree of swelling of the resultant poly( NIPAAm) gels, which was measured by immersing the gels in water at various temperatures ( 0 -50ะC) for 24 h, steeply decreased at about 30ะC with increasing temperature, exhibiting a temperature-responsive character. The gels swelled and shrank in water below and above the temperature, respectively. The extent of the character depended on the concentrations of hydrogen peroxide and monomer. The formation mechanism of the gel in the photopolymerization of NIPAAm using hydrogen peroxide photoinitiator was discussed.